Fix trained agents and reward normalization for hyperparam optim

[araffin]

Description

See araffin/rl-baselines-zoo#87 and #20

Motivation and Context

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Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (update in the documentation)

Checklist:

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